(1) Field of the Invention
The present invention relates to a positive photoresist composition excellent in sensitivity, heat resistance and dimensional reproducibility.
(2) Description of the Prior Art
A light-sensitive resin composition containing a compound having a quinone diazide group such as a naphthoquinone diazide or benzoquinone diazide group, upon irradiation with light of from 300 to 500 nm, produces a carboxyl group by the decomposition of the quinone diazide group, whereby the composition, which is alkali-insoluble in itself, turns alkali-soluble. This property is made use of to use the composition as a positive photoresist. In this case, a novolak resin is generally used in combination. This resin is important to obtain a uniform and strong resist film. This positive photoresist has a feature that the resolving power is markedly superior as compared with a negative photoresist. This high resolving power is made use of to use the positive photoresist as an etching protective film when a photographic etching method is applied to produce copper-clad printed circuit boards for printed circuit and integrated circuits such as IC and LSI.
Particularly, with the integrated circuits, miniaturization has proceeded with a rise in the integration level to result in a demand for the formation of patterns of an order of submicron. Hitherto, a mask contact printing method has been used for the formation of integrated circuits, but it is said that the limit of this method is 2 .mu.m, and a reduction projection exposure method is attracting attention as a substitute therefor. This method is of a form of reducing and projecting the pattern of a master mask (reticule) by passing light through a lens system, and a resolving power of up to an order of submicron can be obtained. However, one of the problems of this method is a low through-put. That is, unlike the 1:1 contact or projection exposure methods such as the conventional mask contact printing method, this reduction projection exposure method, because of a need of divided and repeated light exposure, has a problem that the total of exposure times per a piece of wafer is long.
For solving this problem, raising the sensitivity of the resist used is most important, not to speak of improvement in the apparatus. If the exposure time can be shortened by raising the sensitivity, a rise in the through-put and as a result, a rise in the yield can be attained.
When the positive photoresist now in practical use is viewed from this standpoint, it may not always be said that the photoresist is satisfactory in terms of the sensitivity. Generally, the positive photoresist is lower in the sensitivity than the negative photoresist, an improvement in the sensitivity being desired.
The simplest method to raise the sensitivity is to reduce the molecular weight of a novolak resin used in the positive photoresist, and this method accelerates the dissolution of the photoresist in an alkaline developing solution to apparently raise the sensitivity of the photoresist.
This method, however, causes a very serious disadvantage that the heat resistance of the photoresist lowers, in addition to problems such as: Film thickness loss in the unexposed area becomes large (lowering in the so-called film thickness retention), the shape of the pattern changes for the worse, and the so-called .gamma. value lowers because a difference in the rates at which the exposed area and unexposed area dissolve in the developing solution becomes small.
The term "heat resistance of the positive photoresist" referred to herein means the resistance to heat of the resist pattern after development. More specifically, when heat is applied from the outside to the resist pattern formed on a substrate, the resist pattern softens and flows, or changes in the shape at a temperature higher than a certain one, and what degree of ambient atmospheric temperature the resist pattern can stand to retain its original sharp shape without deformation is the measure of the heat resistance of the photoresist. When the resist pattern softens and flows, or changes in shape, the dimension of the etched substrate after etching goes wrong, or the dimension fluctuates, so that the resist cannot perform a role required for it as an etching protective film. That is, the yield sharply lowers, which is not preferred.
In relation to these techniques, Japanese Patent Publication Kokai (Laid-open) No. 189739/1985 discloses that a photoresist composition superior in the heat resistance is obtained when the content of 1-3 nucleus of a novolak resin is made less than 10% by weight.
On the other hand, three great characteristics required for the resist are sensitivity, resolution and heat resistance.
If the resist lacks any one of these characteristics, it cannot be used as resist, so that all the three characteristics need to be excellent. Among these three, sensitivity and heat resistance tend to conflict with each other in a certain aspect, and resists satisfying these two at the same time have not been on the market up to now.
In view of the present situation like this, the present inventors extensively studied to develop a positive photoresist having any one of the sensitivity and heat resistance improved without impairing the other one as well as having excellent resolution, and as a result, found that a positive photoresist composition having excellent heat resistance can be obtained with a high sensitivity being maintained by adjusting the shape of the gel permeation chromatographic pattern (GPC pattern) of a novolak resin to the predetermined one. The present inventors thus completed the present invention.